This invention relates to the field of holography and, more specifically, to a process for making phase holograms in photographic material.
Phase holograms prepared by conversion of the metallic silver image of a conventionally processed photographic plate to a transparent compound which has a refractive index which differs from that of the gelatin matrix are known. These holograms are prepared by exposure of a photographic plate to a source of coherent light source under hologram forming conditions. More particularly, a coherent light beam is divided and one portion, the reference beam, is directed to the recording medium or photographic plate and the other to the object to be recorded and the reflection or transmission from the object in the form of a so-called object beam is also directed to the recording medium. The resulting interference pattern of the object beam and reference beam is recorded in the recording medium. The photographic plate when developed by conventional means is termed a hologram and when viewed by transmitted or reflected coherent light of the same wavelength directed to the hologram at the same angularity as the reference beam in the hologram formation has the capacity to duplicate and reproduce the original object beam. The silver halide phase hologram differs only in the sense that in a typical photographic hologram the silver particles forming the grating resulting from exposure are opaque. In the phase hologram the silver metal is converted to a silver salt which, although transparent to light, has an index of refraction that differs from that of the emulsion. The grating principle in the formation of the hologram, however, remains the same.
Phase holograms where bleached photographic plates are employed are those where the metallic silver resulting from exposure and remaining in the emulsion after development is converted to a silver halide, such as bromide. Of course, the silver halide salts produced by bleaching a hologram, while altered by the photographic exposure, development and bleaching process and thereby made less sensitive to light than the original salt of the emulsion, are still subject to photodecomposition, especially when exposed to high intensity laser beams. This photodecomposition or reconversion of the silver halide to silver metal results in print-out darkening of the plate or transparency and with that print-out, limits the life of the phase hologram since the silver metal is opaque and blocks the laser beam rather than transmitting and refracting it as originally intended.
To avoid the print-out darkening characteristics of the bleached silver halide process for making holograms, a process using hardened dichromated gelatin has been used. While this type of process produces a hologram with minimal light scatter or noise and no print-out darkening, the process does not have the light sensitivity and spectral response of the bleached silver halide processes and, in addition, the sensitized dichromated gelatin does not have the storage stability and must be used shortly after preparation for best results. To explain, the hardened dichromated gelatin plates are prepared usually from fixed silver halide emulsion. The plates are sensitized by soaking in an ammonium dichromate solution, dried and then exposed. Following exposure the plates are washed in running water to remove the remaining dichromate sensitizer and dehydrated in Isopropanol baths. The phase holograms formed consist of cross linked or tanned gelatin in a gelatin matrix. The difference in refractive indexes between the tanned and the untanned gelatin produces the diffraction that is the basis for the hologram. The absence of print-out darkening is due to the absence of any light sensitive silver salt in the final product. Thus no desensitizing treatment is required. The disadvantage with the silver halide process is that the silver halide crystals in the photographic plates introduce unwanted scattered light and upon reexposure to light are reduced back to silver which degraded the performance of the hologram. It is therefore the principal object of this invention to provide a process for making phase holograms which combines the operational advantages of both the bleached silver halide hologram and the dichromated gelatin hologram which include longer storage shelf life, greater light sensitivity and wide spectral response of the bleached silver halide hologram as well as low scattering noise, high diffraction efficiency and the lack of print out darkening effects of the dichromated gelatin hologram. Another object of this invention is to provide a process for making a phase hologram from a silver halide emulsion which will contain no metallic silver or silver salts or other metal salts or dyes at the end of the process. A further object of this invention is to provide a process for making phase holograms from a silver halide emulsion which will require no desensitizing treatment to prevent print-out darkening of the finished hologram. A further object of this invention is to provide a process for making a phase hologram from a silver halide emulsion in which the metallic silver in exposed areas is converted by the tanning bleach which also acts to harden the gelatin in those exposed areas and in which the silver halide is removed by non-tanning fixing agents.